Allow setting working directory for compile steps

Current working directory affects the linking behavior when `-L` is a
relative path.

We need this to add a test case to `test/link/elf.zig` for checking
`-l`/`-L` behavior compatibility with Clang/GCC for ELF.

Author: a-khabarov

lib/std/Build/Step.zig

@@ -316,6 +316,7 @@ pub fn addError(step: *Step, comptime fmt: []const u8, args: anytype) error{OutO
 /// is the zig compiler - the same version that compiled the build runner.
 pub fn evalZigProcess(
     s: *Step,
+    opt_cwd: ?[]const u8,
     argv: []const []const u8,
     prog_node: std.Progress.Node,
 ) !?[]const u8 {
@@ -324,8 +325,8 @@ pub fn evalZigProcess(
     const arena = b.allocator;
     const gpa = arena;
 
-    try handleChildProcUnsupported(s, null, argv);
-    try handleVerbose(s.owner, null, argv);
+    try handleChildProcUnsupported(s, opt_cwd, argv);
+    try handleVerbose(s.owner, opt_cwd, argv);
 
     var child = std.process.Child.init(argv, arena);
     child.env_map = &b.graph.env_map;
@@ -334,6 +335,7 @@ pub fn evalZigProcess(
     child.stderr_behavior = .Pipe;
     child.request_resource_usage_statistics = true;
     child.progress_node = prog_node;
+    child.cwd = opt_cwd;
 
     child.spawn() catch |err| return s.fail("unable to spawn {s}: {s}", .{
         argv[0], @errorName(err),

lib/std/Build/Step/Compile.zig

@@ -1,5 +1,6 @@
 const builtin = @import("builtin");
 const std = @import("std");
+const Build = std.Build;
 const mem = std.mem;
 const fs = std.fs;
 const assert = std.debug.assert;
@@ -214,6 +215,9 @@ is_linking_libcpp: bool = false,
 
 no_builtin: bool = false,
 
+/// Use `setCwd` to set the initial current working directory
+cwd: ?Build.LazyPath = null,
+
 pub const ExpectedCompileErrors = union(enum) {
     contains: []const u8,
     exact: []const []const u8,
@@ -1739,7 +1743,8 @@ fn make(step: *Step, prog_node: std.Progress.Node) !void {
         try zig_args.append(resolved_args_file);
     }
 
-    const maybe_output_bin_path = step.evalZigProcess(zig_args.items, prog_node) catch |err| switch (err) {
+    const cwd: ?[]const u8 = if (compile.cwd) |lazy_cwd| lazy_cwd.getPath(b) else null;
+    const maybe_output_bin_path = step.evalZigProcess(cwd, zig_args.items, prog_node) catch |err| switch (err) {
         error.NeedCompileErrorCheck => {
             assert(compile.expect_errors != null);
             try checkCompileErrors(compile);
@@ -1982,3 +1987,8 @@ fn moduleNeedsCliArg(mod: *const Module) bool {
         else => continue,
     } else false;
 }
+
+pub fn setCwd(self: *Compile, cwd: Build.LazyPath) void {
+    cwd.addStepDependencies(&self.step);
+    self.cwd = cwd;
+}

lib/std/Build/Step/ObjCopy.zig

@@ -161,7 +161,7 @@ fn make(step: *Step, prog_node: std.Progress.Node) !void {
     try argv.appendSlice(&.{ full_src_path, full_dest_path });
 
     try argv.append("--listen=-");
-    _ = try step.evalZigProcess(argv.items, prog_node);
+    _ = try step.evalZigProcess(null, argv.items, prog_node);
 
     objcopy.output_file.path = full_dest_path;
     if (objcopy.output_file_debug) |*file| file.path = full_dest_path_debug;

lib/std/Build/Step/TranslateC.zig

@@ -154,7 +154,7 @@ fn make(step: *Step, prog_node: std.Progress.Node) !void {
 
     try argv_list.append(translate_c.source.getPath2(b, step));
 
-    const output_path = try step.evalZigProcess(argv_list.items, prog_node);
+    const output_path = try step.evalZigProcess(null, argv_list.items, prog_node);
 
     translate_c.out_basename = fs.path.basename(output_path.?);
     const output_dir = fs.path.dirname(output_path.?).?;